Aqueous suspensions and/or aqueous dispersions and/or dry calcium carbonate mineral pigments of mineral matter are presently used in various applications and, especially when containing calcium carbonate as inorganic particulate material, they are notably used to produce compositions containing pigments or fillers, which can further be used in paper manufacturing and/or paper and/or plastic coatings, as fillers for plastics or in aqueous paint compositions and the like.
In order to maintain such slurries in stable form in terms of viscosity, and to avoid undesirable phenomena such as aggregation, flocculation or sedimentation during slurry manufacturing, transport or storage, the skilled man in the art knows that it is necessary to obtain stable aqueous suspensions and/or aqueous dispersions of particulate material using dispersing and/or grinding aid agents. It is also well known that the choice of such dispersing and/or grinding aid agents may later influence some of the characteristic properties of the final products made based on these slurries; for instance, in the case of aqueous paint, paper or paper coating formulations, the state of the art includes documents concerning dispersing agents that enhance the optical properties of final products, such as opacity, gloss or the scattering coefficient.
U.S. Pat. No. 5,043,017 and U.S. Pat. No. 5,156,719 deal with finely divided calcium carbonate that is acid-stabilized through the use of a calcium-chelating agent, a conjugate base, such as polyphosphates and a weak acid, the latter being preferentially phosphoric acid, and a process in order to obtain such calcium carbonate and a paper containing such calcium carbonate.
These solutions do not comply with new requirements of the skilled man in the art, in terms of non-use of certain chemicals which are now considered as potential pollutants in waste waters and process waters, namely phosphate compounds. In fact, chemical products containing phosphates are currently subject to an increasing number of restrictions. In the Spring of 2003, the European Union signed a Protocol on a Pollutant Release and Transfer Register. On Oct. 7, 2004, the European Commission adopted a proposal for a Regulation for future reporting obligations: COM (2004) 634. Annex I of this proposal lists the industrial facilities and activities under consideration: the mineral industry is mentioned on page 25 (underground mining and related operations, opencast mining). Annex II lists 90 products under consideration: on page 28 is mentioned phosphorus total.
In view of the above, the previous solutions disclosing the use of phosphates and above all of phosphoric acid can not be considered as relevant, efficient and useful for the skilled man in the art.
As known for many years in the art, another solution to obtain stable suspensions of minerals matter is to use of acrylic homopolymers and/or acrylic copolymers with other water-soluble monomers. FR 2 603 042, EP 0 100 947, EP 0 127 388, EP 0 129 329, EP 0 542 644 disclose the use of such polymers exhibiting low molecular weights for this purpose. Although these various types of dispersing agents make it possible to obtain aqueous suspensions of finely divided mineral matter that are stable over time, they do not allow the subsequent suspension or redispersion of mineral matter in water, such as notably calcium carbonate, when issued from a mechanical and/or thermal reconcentration step subsequent to a step of dry and/or wet grinding without the use of dispersant and at low concentration in terms of dry matter (<40% by dry weight), and optionally followed by an additional grinding step. Such low solid ground calcium carbonate, ground in aqueous suspension with using any dispersing and/or grinding aid agents are difficult to disperse to form high solids content suspensions.
Faced with this problem, the skilled man in the art knows WO 01 48093 that teaches the selection of homopolymers of acrylic acid and/or water soluble copolymers of acrylic acid with a high molecular weight (corresponding to viscosity indexes ranging from 0.08 to 0.80). Another solution is proposed in EP 0 850 685, which discloses the use of a water soluble copolymer of acrylic acid and maleic acid, in a certain ratio, and according to a certain molecular weight.
However, the above-mentioned polymers and copolymers are derived from petrochemicals which are well known to contribute to an undesirable increase in TOC in the atmosphere and DOC in water, and do not comply with the Kyoto Protocol, as mentioned previously herein.
Furthermore, none of these documents teach the influence of the dispersing agent used on the final properties of the aqueous formulations containing a suspension of mineral matter obtained according to these inventions and, more particularly, neither deals with the possible enhancement of optical properties of paints or paper or paper coatings manufactured using such slurries: the present invention allows the skilled man in the art to comply with the requirements of the end-user in terms of maintaining or improving the optical properties of the final products.
It is of note that such aqueous solutions containing zirconium compounds have been well known in the art for many years, especially as regards their use as crosslinking agents in aqueous coating formulations as disclosed in “Zirconium compounds find new uses in surface coatings” (Modern paint and coatings, February 1988, 4482, pp 36-39). The fact that polymeric species of zirconium can interact with the functional groups of organic polymers makes them of great interest in water-based inks and coatings where they can improve both heat resistance and scrub resistance: as mentioned in the article “Zirconium compounds in waterbased coatings” (Polymers paint colour Journal, Mar. 9, 1988, 178, 4209, pp 154-162), this is notably the case for Bacote™ 20 (AZC—ammonium zirconium carbonate) and Zirgel™ K (KZC: potassium zirconium carbonate) manufactured by Magnesium Elektron Ltd. It is also well established that some of these compounds may be used in ink-jet coating formulations, where they can influence rheological, optical and printing properties of these inks. These results are discussed relative to ZAA (zirconium carbonate) and AZC in “Influence of cationic additives on the rheological, optical, and printing properties of ink-jet coatings” (Polymers paint colour Journal, Mar. 9, 1988, 178, 4209, pp 154-162). However, all of the above documents refer to the use of zirconium compounds in aqueous formulations containing organic polymers (primarily binders which are strongly crosslinked by the above-mentioned zirconium compounds), which is entirely different from the present invention which deals with the reduction or elimination of organic polymers, more particularly polycarboxylates, used as grinding and/or dispersing aid agents in aqueous suspensions of calcium carbonate.
Finally, although they cannot be considered as being strictly within the application field of the present invention, since they do not address the requirements of the skilled man in the art entirely avoiding the use of phosphate compounds, providing stable slurries with high solids content of more than 45% by dry weight of mineral matter, possibly of more than 65% by dry weight of mineral matter, and possibly of more than 78% by dry weight of mineral matter, maintaining and/or enhancing the optical properties of the final products, and reducing the TOC value for an equal degree of calcium carbonate dispersion), the following documents are mentioned since they disclose the use of zirconium compounds in aqueous suspensions of mineral matter, free of organic polymers, and the enhancement of optical properties of final products such as paper and paper coatings.
U.S. Pat. No. 3,597,251 teaches that either zinc oxide or zirconium oxide or mixtures thereof can be used to improve the dispersion of calcium carbonate in water, thus leading to solids contents ranging from 55% to 80% by dry weight of mineral matter; however, the use of the above mentioned products is restricted to a combination with phosphate dispersants, as indicated in claim 1.
EP 0 206 837 discloses a process of preparing a clay pigment with the use of a sufficient amount of a zirconium ion source to enhance the optical properties of a substrate containing the pigment. AZC is disclosed as one of the most efficient zirconium ion sources to improve brightness, opacity and the scattering coefficient of a paper formulated according to this invention. Nevertheless, in view of the given examples, it clearly appears that, in order to obtain slurries of high solids content (higher than 60% by dry weight of mineral matter), conventional phosphate-based dispersants must be used (see example 1, p 12); the higher the concentration of AZC, the higher the optical properties, the lower the dispersibility of the mineral particles (see example 2, p 15).
Thus, the skilled man in the art obviously deduces from these documents that while zirconium compounds may be used to disperse minerals such as clay, and highly concentrated slurries may be obtained, requiring however the use of phosphate-based dispersants, the mineral's dispersability is decreased: faced with the new regulations concerning pollutants in the mining industry, this is precisely what he wants to avoid.